This invention relates to a separator of a fuel cell having a gas path and a producing method thereof. More particularly, it relates to a separator having an electrically conductive member as a basic material, and a producing method thereof.
There is so far known a fuel cell for generating electricity utilizing a fuel gas. The fuel cell is usually made up of a large number of cells. For further improving performance of the fuel cell, there is a demand raised for reducing fluctuations in an electromotive force and a voltage from one cell to another.
In producing a groove-shaped gas path in a separator of the fuel cell for industrial application, there are so far known a method for cutting a metal plate, which is to be a separator, using a suitable cutting tool, to form a groove-shaped gas path, a method for press-working a metal plate, which is to be a separator, to form a groove-shaped passage or duct, and a method for etching (or electroless chemical etching) a metal plate, which is to be a separator of chemically excavating a groove-shaped gas path. The method of producing the gas path by etching (that is, presumed as electroless chemical etching) is disclosed in JP Patent Kokai JP-A-4-267062.
However, in the course of the investigations toward the present invention the following problems have been encountered. Namely, in the fuel cell, the above-mentioned fluctuations in the electromotive force and the voltage, from one cell to another, are caused by factors such as insufficient uniformity in gas flow containing an active material, insufficient uniformity in catalytic activity of each electrode, insufficient uniformity in electrical contact resistance and insufficient control in temperature distribution. Among other possible factors, there is insufficient uniformity in gas flow caused by depth variations in each separator gas path.
With the conventional method for cutting a gas path by machining with a cutting tool, the gas path is cut only little by little, so that the removal rate and hence productivity is low, even though the depth of the gas path can be precisely controlled by fine control techniques. In particular, if the separator is formed of a hard material which cannot be machined easily, the machining time is prolonged, thus lowering the productivity. In drawing an intricate pattern shape of a complicated gas path, the machining time is prolonged, thus again lowering the productivity. In addition, there is a problem that a cutting tool suffers wear through machining. Moreover, working-transmuted layers, possibly giving rise to non-uniform electrical conductivity, or residual stress layers, possibly giving rise to chronological dimensional changes, are liable to be produced.
In a conventional method for forming a gas path by press-working, the separator is susceptible to springback, such that, even if the fuel cell is assembled to high precision, the gas path and the separator tend to be lowered in dimensional stability due to springback. In a fuel cell comprised of a large number of cells with separators laminated each other, the risk is high that the gas path and the separator are lowered in dimensional stability due to superimposed dimensional changes caused by springback of the respective separators. Moreover, since a pattern configuration of the gas path depends on the press-workability, limitations are imposed by press-workability on designing of the pattern configuration of the gas path. Therefore, in designing the pattern configuration of the gas path, it is necessary to take into account the relative ease in press-working in addition to uniformity in the gas flow. Moreover, if a press dies are used for prolonged time, the gas path depth may be varied due to wear caused to the press dies.
In the conventional method for forming a gas path by electroless chemical etching, the gas path depth tends to be fluctuated. Moreover, a gas path having plural depths, or a gas path having a grandient or gradation on its bottom, cannot be formed easily by electroless chemical etching, and is in need of complicated process steps.
In view of the above-described status of the art, it is an object of the present invention to provide a separator of a fuel cell which may be used with advantage for reducing the fluctuations in the power generation and electromotive force from one cell to another.
It is another object of the present invention to provide a producing method for a separator of a fuel cell with which fluctuations in the gas path depth can be reduced, the problem of wear to the machining electrodes can be eliminated, the gas path can be formed easily with an acceptable removal rate even if the separator is formed of a hard material difficult-to-cut, and with which the gas path can be produced easily with plural depths and a desired gradation.
Further objects of the present invention will become apparent in the entire disclosure.
For achieving the above objects, the present inventors have conducted perseverant researches, and found that, if a surface roughness of the recessed gas path of the separator is prescribed to an extremely small value, such as to a value not larger than 1 xcexcm in terms of Rz, the gas flow containing an active material in the gas path is smoothed and uniformed thus advantageously reducing fluctuations in the power generation and electromotive force (voltage) from one cell to another.
In particular, the present inventors have found that, if the recessed gas path is of a shallow depth, such as 2 mm or less or 10 mm or less, the surface roughness of the bottom of the gas path significantly influences the gas flow, and that, if the surface roughness of the bottom of the gas path is prescribed to 1 xcexcm or less in terms of Rz, the gas flow containing the active material is further smoothed and uniformed thus advantageously reducing fluctuations in the power generation and electromotive force from one cell to another. This finding has led to completion of the separator of the present invention.
It is noted that, in a separator adapted for cutting a groove-shaped gas path by machining with a cutting tool, the gas path presents significant surface roughness. In a separator in which a groove-shaped gas path is formed by press-bend-working, micro-sized irregularities tend to be produced under the effect of buckling due to compressive stress at the time of press-bend-working or tensile deformation caused by the tensile stress. In a separator in which a groove-shaped gas path is formed by electroless chemical etching, etching pits tend to be produced to increase the surface roughness.
In one aspect, the present invention provides a separator of a fuel cell defining a recessed gas path in which flows a gas containing an active material, wherein a bottom surface of the gas path has a surface roughness not larger than 1 xcexcm in terms of Rz.
The separator of the present invention is effective to smooth the gas flow containing an active material.
The present inventors have conducted perseverant researches into forming a gas path of a separator of a fuel cell, and found that, for accomplishing the above object, it is meritorious to produce the gas path by electrolytic processing. The present inventors have confirmed this by experiments, and have arrived at the present invention.
Thus, in another aspect, the present invention provides a method for producing a separator of a fuel cell having a recessed gas path in which flows a gas containing an active material. The method includes (a) a step of providing an electrically conductive member which later serves as a separator, and a processing electrode having electrode projections shaped complementarily to a pattern shape of the gas path, (b) a step of placing the electrically conductive member so as to face the electrode projections of the processing electrode, and (c) a step of supplying current to an area between the electrically conductive member and the processing electrode, in a state in which an electrolytic solution is interposed between the electrically conductive member and the electrode projections of the processing electrode, so as to dissolve the electrically conductive member to excavate the gas path in the electrically conductive member.
When the current is supplied between the electrically conductive member and the processing electrode, electrolytic processing of electro-chemically dissolving the electrically conductive member takes place to excavate a groove-shaped gas path in the surface of the electrically conductive member.
The meritorious effect of the present invention are summarized as follows.
With the separator of the present invention, the surface roughness of the bottom of the recessed gas path is prescribed to an extremely small value, thus aiding in smoothing and uniforming the gas flow in the gas path containing the active material.
In particular, if the recessed gas path is of shallow depth such that the gas pathage resistance tends to be affected by surface roughness, the separator of the present invention promotes smoothing and uniforming the gas flow.
Thus, the present separator is meritorious in reducing fluctuations in power generation and electromotive force in each cell to contribute to improved performance of the fuel cell.
Moreover, since the gas path is smooth, expectations may be had of improving corrosion resistance in the gas path to promote smoothing and uniforming the gas stream.
With the present method, it is possible to reduce depth fluctuations in the gas path and hence to minimize the gas path depth fluctuations even if the number of produced separators is increased. The result is that, even if a large number of cells are layered together to constitute a fuel cell, the gas flow containing an active material can be uniformed with advantage in each cell. The result is that fluctuations in power generation or electromotive force in each cell can be reduced with advantage to contribute to improved performance of the fuel cell.
With the inventive method, as distinct from the method of forming the gas path by press-working in constituting the gas path, no significant load is applied to the processing electrode, so that the processing electrode itself is consumed only to a lesser extent. Thus, the processing electrode, once produced, can be used over an extended period of time, such as semi-permanently. The result is that fluctuations in the gas path caused by wear to the processing electrode are minimized.
Moreover, in the present method, exploiting electrolytic processing, it is possible to evade or reduce the influence of hardness of the electrically conductive member as a separator in preparing the gas path, so that the gas path can be formed in a high precision even in case of the hard electrically conductive member. Therefore, the present method can be applied with advantage to the forming of a gas path in an electrically conductive member formed of an alloy material, which is superior in corrosion resistance and therefore is desirable as a separator material but which is not necessarily easy to press-work or cut, such as stainless steel.
In the method of the present invention, the protruding amount of the electrode projections of the processing electrode can be adjusted depending on a desirable site in the gas path. In this case, the gas path can be changed in depth depending on the desirable site in the gas path. That is, the gas path having plural depth values or a gradation can be produced more easily in one step than if the gas path is formed by electroless chemical etching.